The present invention relates to high-purity alkanolamines and their production process. More specifically the present invention relates to a process to produce high-purity alkanolamines, whose metal content is very small, particularly iron content is less than 50 ppb.
Alkanolamines are used for a wide variety of application, for example, as intermediates for surface-active gents, agricultural chemicals, pharmaceuticals etc. or as cleaning agent for surfaces of metal,glass etc. Particularly monoethanolamine shows high peeling and cleaning properties to photoresist for semiconductor, liquid crystalline substance, solder flux, fat and oil etc. and is preferable as photoresist peeling agent, cleaning agent for liquid crystal cells, cleaning agent for solder flux, cleaning agent for degreasing of metallic products, cleaning agent for electronic parts, etc. For using for such applications, however, monoethanolamine of high purity, particularly with small metal content, is required (cf. Japanese Laid-Open Patent Publication No. 271699/1993).
By using conventional alkanolamine production facilities, however, it was difficult to produce monoethanolamine with small metal content due to its own strong metal-corrosive property (cf. Reference Example 1 given later).
One of the purposes of the present invention is to provide high-purity alkanolamines, whose iron content is less than 50 ppb Another purpose of the present invention is to provide a process to produce high-purity alkanolamines, particularly alkanolamines with very small metal content, by using conventional alkanolamine production facilities with only a simple improvement.
Crude alkanolamine, for example, a reaction product, obtained by the reaction of ethylene oxide and ammonia and consisting of a mixture of monoethanolamine, diethanolamine and triethanolamine, is separated into each component by distillation for each application. According to the research of the present inventors it was found out that the metal content, particularly iron content, increases during distillation, particularly in the overhead product line at the top of the distillation tower, and its cause is the dissolution of metal components of materials such as stainless steel or iron, of which the piping is made. The present inventors, therefore, have succeeded in obtaining alkanolamines with very small metal content by covering with an alkanolamine-resistant material the inner wall of equipment that contacts alkanolamines (for example, piping etc.) after the top of the distillation tower. Thus, according to the present invention, there are provided high-purity alkanolamines, whose iron content is less than 50 ppb, and a production process of a high-purity alkanolamine characterized by covering with an alkanolamine-resistant material at least a part of the inner wall of equipment that contacts alkanolamines from the top of the distillation tower to the storage tank, in producing a high-purity alkanolamine by distillation of a crude alkanolamine.
According to the present invention, the dissolution of metal components in the equipment material into alkanolamines is prevented by covering at least a part of the inner wall of equipment with an alkanolamine-resistant material, for example, resin, glass etc., preferably resin. As the above-mentioned resin, an alkali-resistant resin is preferable and there can be used, for example, polyethylene resin, polypropylene resin, polystyrene resin, fluororesin etc. The covering thickness can be suitably chosen to such an extent as to prevent the dissolution of metal components.
As representative examples of the alkanolamines of the present invention there can be mentioned monoethanolamine, diethanolamine, triethanolamine etc. Above all, monoethanolamine is preferably used. The present invention, therefore, will be described in mentioning monoethanolamine as example.
Monoethanolamine is obtained by separation through distillation from a reaction mixture containing monoethanolamine, diethanolamine, triethanolamine etc. obtained by the reaction of ethylene oxide and ammonia. In the present invention, at least a part of the inner wall of equipment that contacts monoethanolamine from the top of the distillation tower to the storage tank is covered with an alkanolamine-resistant material such as resin. The distillation tower mentioned here includes a distillation tower to separate monoethanolamine from a reaction mixture and a distillation tower to purify the separated monoethanolamine. And the equipment that contacts monoethanolamine from the top of the distillation tower to the storage tank includes the piping to transfer the distillate from the top of the distillation tower to the condenser, the piping to reflux a part of the condensate to the distillation tower, the piping to transfer the condensate to the next distillation tower, the piping to transfer the condensate as the product ethanolamine to the storage tank, and the storage tank. In the present invention, it is possible to cover all the inner wall of the above-mentioned piping and the storage tank and it is possible, too, to cover only the inner wall of the piping, excluding the storage tank.